Multi-user detection using equalization and successive interference cancellation

ABSTRACT

A method and apparatus for multi-user detection is disclosed. A signal is received in a shared spectrum, and samples of the received signals are produced as a received vector. The received vector is segmented into vector segments. Each segment has a portion that overlaps with another segment and the overlapping portion includes at least one chip less than twice a channel impulse response length. For each vector segment, symbols are successively determined for communications by determining symbols for a communication in the communications, ordering the communications by received power and removing a contribution of the communication from the vector segment. The determining of symbols includes equalizing an input vector corresponding to a segment of the received vector using fast Fourier transform. The determined symbols are assembled into a data vector for each communication in the communications.

CROSS REFERENCE TO RELATED APPLICATION(S)

This application is a continuation of U.S. patent application Ser. No.12/547,028, filed Aug. 25, 2009, which is a continuation of U.S. patentapplication Ser. No. 12/049,806, filed Mar. 17, 2008, now U.S. Pat. No.7,593,461 issued Sep. 22, 2009, which is a continuation of U.S. patentapplication Ser. No. 10/748,544, filed Dec. 30, 2003, now U.S. Pat. No.7,346,103, issued Mar. 18, 2008, which in turn claims priority from U.S.Provisional Application No. 60/451,591, filed Mar. 3, 2003, which isincorporated by reference as if fully set forth.

FIELD OF INVENTION

The invention generally relates to wireless communication systems. Inparticular, the invention relates to detection of multiple user signalsin a wireless communication system.

BACKGROUND

A typical wireless communication system includes base stations whichcommunicate with wireless transmit/receive units (WTRUs). Each basestation has an associated operational area where it communicates withWTRUs which are in its operational area. In some communication systems,such as code division multiple access (CDMA), multiple communicationsare sent over the same frequency spectrum. These communications aretypically differentiated by their codes.

Since multiple communications may be sent in the same frequency spectrumand at the same time, a receiver in such a system must distinguishbetween the multiple communications. One approach to detecting suchsignals is matched filtering. In matched filtering, a communication sentwith a single code is detected. Other communications are treated asinterference. To detect multiple codes, a respective number of matchedfilters are used. These signal detectors have a low complexity, but cansuffer from multiple access interference (MAI) and inter-symbolinterference (ISI).

Other signal detectors attempt to cancel the interference from otherusers and the ISI, such as parallel interference cancellers (PICS) andsuccessive interference cancellers (SIGs). These receivers tend to havebetter performance at the cost of increased complexity. Other signaldetectors detect multiple communications jointly, which is referred toas joint detection. Some joint detectors use Cholesky decomposition toperform a minimum mean square error (MMSE) detection and zero-forcingblock equalizers (ZF-BLEs). These detectors tend to have improvedperformance but high complexities.

Accordingly, it is desirable to have alternate approaches to multi-userdetection.

SUMMARY

A method and apparatus for multi-user detection is disclosed. A signalis received in a shared spectrum, and samples of the received signalsare produced as a received vector. The received vector is segmented intovector segments. Each segment has a portion that overlaps with anothersegment and the overlapping portion includes at least one chip less thantwice a channel impulse response length. For each vector segment,symbols are successively determined for communications by determiningsymbols for a communication in the communications, ordering thecommunications by received power and removing a contribution of thecommunication from the vector segment. The determining of symbolsincludes equalizing an input vector corresponding to a segment of thereceived vector using fast Fourier transform. The determined symbols areassembled into a data vector for each communication in thecommunications.

BRIEF DESCRIPTION OF THE DRAWING(S)

FIG. 1 is a simplified diagram of a equalization successive interferencecanceller (EQ-SIC) receiver.

FIG. 2 is an illustration of a preferred segmentation of a receivedvector r.

FIG. 3 is a simplified diagram of an EQ-SIC device.

FIG. 4 is a flow chart for an EQ-SIC receiver.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

The preferred implementation of the preferred embodiments is in afrequency division duplex (FDD) mode of the third generation partnershipproject (3GPP) wideband code division multiple access (W-CDMA)communication system. However, the preferred embodiments can be appliedto a variety of wireless communication systems.

The preferred embodiments can be utilized at a wireless transmit/receiveunit (WTRU) or a base station. A WTRU includes but is not limited to auser equipment, mobile station, fixed or mobile subscriber unit, pager,or any other type of device capable of operating in a wirelessenvironment. A “base station” includes but is not limited to a basestation, Node B, site controller, access point or other interfacingdevice in a wireless environment. Additionally, the preferredembodiments can be applied to WTRUs communicating with each other.

FIG. 1 is a simplified diagram of a preferred equalization/successiveinterference cancellation (EQ-SIC) receiver. Preferably, most of thecomponents shown in FIG. 1, excluding the antenna 20, are implemented asa single integrated circuit. Alternately, the individual components canbe discrete components or a mixture of integrated circuit(s) and/ordiscrete components.

Multiple communications are received by an antenna 20 or antenna arrayof the receiver. A sampling device 22, such as a single or multipleanalog to digital converters (ADCs), samples the received signal toproduce a received vector, r.

The received vector is processed by a segmentation device 24 to producesegments, r₁. . . r_(n) of the received vector r. FIG. 2 is anillustration of a preferred segmentation scheme, although others may beused. As illustrated in FIG. 2, the received vector r is separated intoa plurality of segments, r₁. . . r_(n), (only segments r₁, r₂, r₃, r₄,r₅, r₆, r₇, r₈ and r₉ shown). Preferably, the segments overlap as shown.The amount of the overlap is preferably twice the length the impulseresponse less one chip, 2*(W−1). W is the maximum length of the channelimpulse response, over all channels of all users. This overlapfacilitates the equalization of all chips, even though segments havefinite length. For a given segment, all of the chips contributing to theportion of interest for that segment are equalized. To illustrate, theportion of interest of r₂ is bounded by the dashed lines. The last chipin that portion will extend into the next segment by W−1 chips.Conversely, the chip furthest prior to the first chip in the region ofinterest extending into that region is W−1 chips prior to the firstchip. Accordingly, all chips contributing to the portion of interest andnot in that portion can be equalized, effectively removing theircontribution from the portion of interest.

Although the overlap is shown as being roughly twice the impulseresponse, larger overlaps may be used. The larger overlaps may be usefulbased on the exact receiver implementations. In one embodiment, theEQ-SIC device may use a prime factor algorithm (PFA) fast Fouriertransform (FFT) based implementation. The overlap may be extended toreach a desired optimal PFA or FFT length. In other implementations, theoptimal non-overlap portions may vary based on the signals beingprocessed. To illustrate, in the time division duplex (TDD) mode of 3GPPW-CDMA, based on the burst type, the length of the data field may vary.As a result, the optimum segment length for one burst may not be optimumfor another burst. To utilize one uniform hardware configuration a setsize for a segment may be implemented. Different overlaps may be used tofacilitate the different burst lengths.

A channel estimation device 26 estimates the channel response for eachof the received user signals. Typically, the channel response isestimated using a reference signal, such as a pilot code or a midamblesequence, although other techniques may be used. The estimated channelresponses are represented in FIG. 1 as a channel response matrix H.

FIG. 3 is an illustration of a preferred EQ-SIC device 28 applied to areceived vector segment r_(i). EQ-SIC device 28 includes equalizers 34₁, 34 ₂, . . . , 34 _(K) for equalizing vector segments r_(i), x_(i1),x_(iK−1) configured to produce spread data vectors s_(i1), s_(i2), . . ., s_(iK), respectively. EQ-SIC device 28 also includes despreaders 36 ₁,36 ₂, . . . , 36 _(K) for despreading the spread data vectors s_(i1),s_(i2), . . . , s_(iK), configured to produce soft symbols and harddecision devices 38 ₁, 38 ₂, . . . , 38 _(K) configured to produce hardsymbols vectors d_(i1), d_(i2), . . . , d_(iK) from the respective softsymbols. EQ-SIC device 28 also includes interference constructiondevices 40 ₁, 40 ₂, . . . for determining respective user contributionsr_(i1), r_(i2) . . . in each corresponding spread data vector s_(i1),s_(i2), . . . and subtractors 42 ₁, 42 ₂ for subtracting respective usercontributions r_(i1), r_(i2) . . . from respective corresponding vectorsegments r_(i), x_(i1) . . . In one implementation, all of the usersignals are ranked, such as by their received power. For the user havingthe highest received power, the received vector segment r_(i) isequalized by a equalizer 34 ₁ using the channel response associated withthat user (user 1), producing a spread data vector s_(i1). The codesused by that user signal are used to produce soft symbols of that userdata by a despreader 36 ₁. Hard decisions are performed on that user'ssoft symbols by a hard decision device 38 ₁ to produce a hard symbolvector, d_(i1). Using the detected hard symbols, the contribution ofuser 1 to the spread data vector is determined, r_(i1), by interferenceconstruction device 40 ₁. The user 1 contribution is subtracted from thesegment by a subtractor 42 ₁ producing a new segment x_(i1) having user1's contribution removed. Similar processing is performed on a seconduser (user 2) having a second highest received power level. User 2'shard symbols, d_(i2), are detected using an equalizer 34 ₂, producingspread data vector s_(i2), despreader 36 ₂ and hard decision device 38₂. The contribution of user 2 to x_(i1), r_(i2), is removed using aninterference construction device 40 ₂ and a subtractor 42 ₂. Thisprocedure is repeated K−1 times to produce segment x_(iK−1) which isvector r_(i) with the contributions of K−1 users removed. For the K^(th)user, only the hard symbols d_(iK) are determined using an equalizer 34_(K), producing spread data vector s_(iK), despreader 36 _(K) and harddecision device 38 _(K).

If the EQ-SIC receiver is used at a base station, typically, the hardsymbols from all of the users signals are recovered. However, at a WTRU,the WTRU EQ-SIC receiver may only have one user's signal of interest. Asa result, the successive processing of each user can be stopped afterthe hard symbols of that user of interest's signals are recovered.

Although the previous description detected each user's signalsseparately, multiple users signals may be recovered jointly. In such animplementation, the users would be grouped by received signal power. Thesuccessive processing would be performed on each group, in turn. Toillustrate, the first groups data would be detected and subsequentlycanceled from the received segment, followed by the second group.

After the data for each user in a segment is detected, the data vector,such as d_(i), is stored by a segment storage device 30. To reduce thestorage size, preferably, the segment is truncated to remove portionsnot of interest, only leaving the portion of the segment of interest. Asegment reassembly device 32 produces a data vector, d, having the datafrom all the segments, typically by serially combining the data for eachuser for each segment. To illustrate, the data from user 1 for segment1, d₁₁, is serially combined with the data from user 1 for segment 2,d₁₂.

FIG. 4 is a flow chart for an EQ-SIC receiver. Initially, a receivedvector r is produced, step 50. A channel estimation is performed for allthe users, step 52. The received vector is segmented, r₁ . . . r_(n),step 54. Each segment is processed, step 56. For an i^(th) segment, auser having the highest received power is determined, step 58. Thereceived vector is equalized for that user, step 60. The resultingspread vector is despread using that user's code, step 62. Harddecisions are performed on the despread data, step 64. The contributionof that user to the received vector is determined, step 66. That user'scontribution is subtracted from the received vector, step 68. The nexthighest received power user is processed by repeating steps 60-68, usingthe subtracted received vector as the received vector in those steps,step 70. Store the results for that segment and repeat steps 58-70 foreach remaining segment, step 72. Assemble the stored segments into thedata vector d, step 74. The rate at which channel estimates are made orupdated can vary between different implementations, as the rate ofupdated depends on the time varying nature of the wireless channels.

Preferably, the equalization for each stage of the EQ-SIC device 28 isimplemented using FFT, although other implementations may be used. Onepotential implementation is as follows. Each received segment can beviewed as a signal model per Equation 1.

r_(i)=H_(s)+n  Equation 1

H is the channel response matrix. n is the noise vector. s is the spreaddata vector, which is the convolution of the spreading codes, C, for theuser or group and the data vector, d, for the user or group, as perEquation 2.

s=C d  Equation 2

Two approaches to solve Equation 3 use an equalization stage followed bya despreading stage. Each received vector segment, r_(i), is equalized,step 54. One equalization approach uses a minimum mean square error(MMSE) solution. The MMSE solution for each extended segment is perEquation 4A.

ŝ_(i)=(H_(s) ^(H)H_(s)+σ²I_(s))⁻¹H_(s) ^(H)r_(i)  Equation 4A

σ² is the noise variance and I_(s) is the identity matrix for theextended matrix. (·)^(H) is the complex conjugate transpose operation orHermetian operation. The zero forcing (ZF) solution is per Equation 4B

ŝ_(i)=(H_(s) ^(H)H_(s))⁻¹H_(s) ^(H)r_(i)  Equation 4B

Alternately, Equations 4A or 4B is written as Equation 5.

ŝ_(i)=R_(s) ⁻¹H_(s) ^(H)r_(i)  Equation 5

R_(s) is defined per Equation 6A corresponding to MMSE.

R_(s)=H_(s) ^(H)H_(s)+σ²I_(s)  Equation 6A

Alternately, R_(s) for ZF is per Equation 6B.

R_(s)=H_(s) ^(H)H_(s)  Equation 6B

One preferred approach to solve Equation 5 is by a fast Fouriertransform (FFT) as per Equations 7 and 8, an alternate approach to solveEquation 5 is by Cholesky decomposition.

R_(s)=D_(Z) ⁻¹ΛD_(z)=(1/P) D_(z)*ΛD_(z)  Equation 7

R_(s) ⁻¹=D_(z) ⁻¹Λ⁻¹D_(z)=(1/P) D_(z)*Λ*D_(z)  Equation 8

D_(Z) is the Z-point FFT matrix and Λ is the diagonal matrix, which hasdiagonals that are an FFT of the first column of a circulantapproximation of the R_(s) matrix. The circulant approximation can beperformed using any column of the R_(s) matrix. Preferably, a fullcolumn, having the most number of elements, is used.

In the frequency domain, the FFT solution is per Equation 9.

$\begin{matrix}{{{F\left( \underset{\_}{\hat{s}} \right)} = \frac{\sum\limits_{m = 1}^{M}{{F\left( {\underset{\_}{h}}_{m} \right)}^{*} \otimes {F\left( {\underset{\_}{r}}_{m} \right)}}}{F\left( \underset{\_}{q} \right)}}{{{{where}\mspace{14mu} {F(x)}} = {\sum\limits_{n = 0}^{P - 1}{{x(n)}^{{- j}\; \frac{2\pi \; k\; n}{N}}}}},{{{where}\mspace{14mu} k} = 0},1,\ldots \mspace{14mu},{P - 1}}} & {{Equation}\mspace{14mu} 9}\end{matrix}$

is the kronecker product. M is the sampling rate. M=1 is chip ratesampling and M=2 is twice the chip rate sampling.

After the Fourier transform of the spread data vector, F(ŝ), isdetermined, the spread data vectors ŝ is determined by taking an inverseFourier transform.

1. A method for wireless communication, the method comprising: receivinga signal in a shared spectrum; producing samples of the received signalas a received vector; segmenting the received vector into a plurality ofvector segments, wherein each segment has a portion that overlaps withanother segment and the overlapping portion includes at least one chipless than twice a channel impulse response length; for each vectorsegment, successively determining symbols for a plurality ofcommunications by determining symbols for a communication in theplurality of communications, ordering the communications by receivedpower and removing a contribution of the communication from the vectorsegment, wherein the determining of symbols includes equalizing an inputvector corresponding to a segment of the received vector using fastFourier transform; and assembling the determined symbols into a datavector for each communication in the plurality of communications.
 2. Themethod of claim 1 wherein the determining symbols for a communicationfurther includes despreading the equalized vector segment and making ahard decision on the despread equalized vector segment.
 3. The method ofclaim 1, wherein despreading the equalized vector segment includesapplying a code associated with the communication to produce a despreadequalized vector segment including a plurality of soft symbols.
 4. Themethod of claim 1, wherein removing a contribution of the communicationincludes subtracting the determined symbols from the vector segment. 5.The method of claim 1, wherein successively determining symbols for aplurality of communications includes determining symbols for acommunication of interest.
 6. A wireless transmit/receive unit (WTRU)comprising: an antenna configured to receive a signal in a sharedspectrum; a sampling device configured to produce samples of thereceived signal as a received vector; a segmentation device configuredto segment the received vector into a plurality of vector segments,wherein each segment has a portion overlapping with another segment andthe overlapping portion includes at least one chip less than twice achannel impulse response length; a equalization and successiveinterference canceller configured to successively determine for eachvector segment symbols for a plurality of communications by determiningsymbols for a communication in the plurality of communications, orderingthe communications by received power and removing a contribution of thecommunication from the vector segment, wherein the equalization andsuccessive interference canceller is configured to determine the symbolsby equalizing an input vector corresponding to a segment of the receivedvector using fast Fourier transform; and a segment reassembly deviceconfigured to assemble the determined symbols into a data vector foreach communication in the plurality of communications.
 7. The WTRU ofclaim 6, wherein the equalization and successive interference cancellerfurther includes a despreader configured to despread the equalizedvector segment and a hard decision device configured to make harddecisions on the despread equalized vector segment.
 8. The WTRU of claim7, wherein the despreader is configured to produce a despread equalizedvector segment including a plurality of soft symbols by apply a codeassociated with the communication to the equalized vector segment. 9.The WTRU of claim 6, wherein the equalization and successiveinterference canceller is configured to remove a contribution of thecommunication from the vector segment by subtracting the determinesymbols from the vector segment.
 10. The WTRU of claim 6, wherein theequalization and successive interference canceller is configured todetermining symbols for a communication of interest.